From The Collaborative International Dictionary of English v.0.48 [gcide]:
Wellhole \Well"hole`\, n.
[1913 Webster]
1. (Arch.)
(a) The open space in a floor, to accommodate a staircase.
(b) The open space left beyond the ends of the steps of a
staircase.
[1913 Webster]
2. A cavity which receives a counterbalancing weight in
certain mechanical contrivances, and is adapted also for
other purposes. --W. M. Buchanan.
[1913 Webster]